The present invention relates to a hydrogen refinement apparatus, which refines a reformed gas containing hydrogen as the main component and, in addition, CO and provides a hydrogen gas of high purity.
As the hydrogen source for a fuel cell, a reformed gas obtained by reforming hydrocarbons, alcohols, ethers and the like. In the case of a solid polymer type fuel cell operating at a lower temperature of 100° C. or less, there is a fear that a Pt catalyst used in an electrode is poisoned by carbon monoxide (CO) contained in the reformed gas. When a Pt catalyst is poisoned, the reaction of hydrogen is disturbed and the power generation efficiency of the fuel cell decreases remarkably, therefore, it is necessary to lower the concentration of CO to 100 ppm or less, preferably 10 ppm or less.
Usually, for removal of CO, CO and water vapor are subjected to a shift reaction for conversion into carbon dioxide and hydrogen, in a CO shifting part equipped with a CO shifting catalyst body. By this conversion, the CO concentration can be reduced to a concentration of about several thousands ppm to about 1%. Then, CO is removed to about 10 ppm level causing no adverse influence on the fuel cell, by adding a slight amount of air and utilizing a CO selective oxidation catalyst body.
Herein, for sufficient removal of CO, it is necessary to charge oxygen in an amount of 1 to 3-fold of the amount of CO to the above-mentioned CO selective oxidation catalyst body, and hydrogen is also consumed in an amount corresponding to the amount of oxygen. That is, when the CO concentration is high, the amount of oxygen to be added also increases and the amount of hydrogen consumed increases, leading to significant reduction in the efficiency of the whole apparatus. Therefore, it is necessary to decrease sufficiently CO in the CO shifting part.
Conventionally, there have been used, as the CO shifting catalyst, copper-zinc-based catalysts, copper-chromium-based catalysts and the like for lower temperatures, which can be used at 150 to 300° C., and iron-chromium-based catalysts and the like for higher temperatures, which can function at 300° C. or more. Further, there have been used CO shifting catalysts singly, or combinations of CO shifting catalysts for higher temperatures and for lower temperatures, depending on use conditions for chemical plants and hydrogen generating apparatuses for fuel cell.
When the above-mentioned copper-based CO shifting catalysts for lower temperatures are used as the main catalyst, extremely high catalytic activity is obtained, however, there is a need to perform reduction treatment before use to give activation. Further, due to heat generation during the activation treatment, the treatment has to be conducted over a long period of time while controlling the feeding amount of a reduction gas, for example, so that the temperature of the catalyst does not exceed the heat-resistant temperature. Moreover, there is a possibility that a CO shifting catalyst once activated is oxidized again and deteriorated when oxygen is introduced into the apparatus in stopping and the like. Therefore, countermeasures are required such as prevention of oxidation. Also, since the CO shifting catalyst for lower temperatures is inferior in heat-resistance, and the catalyst can not be heated steeply or acutely in starting the apparatus, it is required to increase the temperature gradually.
On the other hand, when only the CO shifting catalyst for higher temperatures is used, heating in starting has no difficulty, since there is no problem when the temperature increases somewhat excessively due to high heat-resistance thereof. However, since the CO shifting reaction is an equilibrium reaction depending on temperature, when a CO shifting catalyst functioning only at higher temperatures is used, it is difficult to control the CO concentration of a shifted gas to 1% or less. In addition, there is also a problem that purification efficiency lowers in a CO purification part connected to the downstream of the CO shifting part.
As described above, since a longer time is necessary for activation of a shifting part in a hydrogen generating apparatus and handling of the shifting part is complicated in conventional technologies, a hydrogen generating apparatus suitable for frequent repetition of starting-up and stopping cannot be provided.
Accordingly, an object of the present invention is to solve the above-mentioned problems and, in concrete, to provide a hydrogen refinement apparatus, in which activation treatment of a CO shifting catalyst is easy and influences by oxygen introduction during the repetition of starting-up and stopping of operation are decreased, and which operates stably for a longer period of time.